This invention relates to an apparatus mounted on a tractor or trailer dolly for locking a towed-trailer in rigid longitudinal alignment with the tractor or dolly frame.
Trailers in the trucking industry typically include two longitudinal side frame struts on which the trailer bed is secured. The rear axle and wheels are rigidly secured in perpendicular relationship to the trailer frame. The front end of the trailer is usually supported either by a "fifth wheel" assembly of a tractor (semitrailer) or a front axle dolly and wheels secured to the trailer frame through a turntable assembly (full trailer). In either case, the front end of the trailer articulates about the vertical axis of either the "fifth wheel" of the tractor or the turntable of the front axle dolly. During braking maneuvers, or when operating on slippery roads or when backing such semi- and full trailers, the trailer may articulate sharply, and in most cases unexpectedly, about the vertical axis of the fifth wheel or turntable, a phenomenon popularly termed "jackknifing".
Prevention of jackknifing is a frequently addressed problem in the trucking industry. For example, U.S. Pat. Nos. 4,067,592, 3,774,941, 3,556,560, 3,101,959, and 2,499,013 each describe friction breaking devices in combination with "fifth wheel" and/or trailer turntable assemblies. The theory behind the friction breaking devices is to increase resistance to articulation of the trailer about the vertical axis or the fifth wheel or turntable apparatus during maneuvers likely to induce jackknifing. However, as pointed out in U.S. Pat. No. 3,774,941 it is impractical to assume that a frictional breaking device can prevent jackknifing because of the tremendous loads involved. Specifically, the trailer bed itself effectively provides a very long lever arm relative to the vertical axis of the fifth wheel, a factor which favors jackknifing, whereas the breaking mechanisms have an effectively short lever arm for resisting articulation of the trailer about the vertical axis.
Under dynamic load conditions, with a fully loaded trailer, such frictional breaking devices simply cannot prevent a trailer from articulating relative to the tractor or front axle dolly .
In fact, where a loaded trailer is more massive than the tractor or the front axle dolly the problem is not to prevent articulation of the trailer relative to the tractor or front axle dolly, but rather to prevent the tractor or front axle dolly from articulating relative to the trailer.
In less dynamic load conditions such as those experienced in backing conditions, again the mechanical advantage provided by the trailer bed is sufficiently great to overcome any resistance to jackknifing provided by breaking mechanisms.
Finally, frictional breaking mechanisms contemplate substantial modification of most fifth wheel and turntable assemblies and accordingly cannot be adapted to existing fifth wheel and turntable assemblies.
Another approach to prevent jackknifing is suggested in U.S. Pat. Nos. 3,031,205 and 2,468,705 in which mechanical latches mounted on the tractor or dolly frame lock into openings provided in the apron plate of the trailer to prevent articulation of the trailer. The disadvantage of such latch mechanisms as suggested relate first to the lack of strength in the latching mechanisms and second to the fact that registry is required between the latch and the receiving receptacle in the apron plate of the trailer. Specifically, the latch and supporting structures must be extremely strong to be able to resist the tremendous mechanical advantage of the trailer bed under dynamic load conditions. Also, because the latches must engage corresponding apertures in the apron plate of the trailer registry between the aperture and locking lug is required. Finally, the mechanical locking mechanism suggested in the latter references may effectively lock only after the tractor and trailer are in a "jackknife" position.